Guide structure for control elements

ABSTRACT

A guide structure for aligning fuel assemblies forming a nuclear core of a nuclear reactor and for guiding a plurality of control elements for telescoping movement within guide channels of fuel assemblies. Two spaced tube sheets, rigidly connected by a plurality of hollow tubes extending therebetween, are supported by a support means relative to the nuclear core with each of the hollow tubes being in alignment with a guide channel of the fuel assemblies. A plurality of yoke means, each of which interconnects at least two of a plurality of control elements situated within some of the hollow tubes, are provided whereby the interconnected control elments telescopingly move as a unit within the guide channels.

White States atent 1191 i 3,849,257 Bevilacqua Nov. 19, 1974 [5 GUIDESTRUCTURE FOR CONTROL 3,607,629 9/1971 Frisch et a1. 176 36 ELEMENTS3,625,816 12/1971 Aleite et a1. 176/36 [75] Inventor: Frank Bevilacqua,Windsor, Conn. FQREIGN PATENTS OR APPLICATIONS ssig e Comb stion gin ingI c Great Bl'ltalfl Windsor, Conn.

Primary Examiner-Harvey E. Behrend [22] Flled: l 1972 Attorney, Agent,or Firm-John R. Nelson [21] Appl. No.: 266,858

[57] ABSTRACT [52] U.S. Cl. 176/36 R, 176/36 S, 176/86 R, A guidestructure for aligning fuel assemblies forming 176/87, 285/20 a nuclearcore of a nuclear reactor and for guiding a [51] Int. Cl G216: 7/08plurality of control elements for telescoping move- [58] Ei elQ qfSearch ..176/36, 86, 50,337, 61, ment within guide channels of fuelassemblies. Two 176/64; 285/19, 20 spaced tube sheets, rigidly connectedby a plurality of hollow tubes extending therebetween, are supported[56] References Cited by a support means relative to the nuclear corewith UNITED STATES PATENTS each of the hollow tubes being in alignmentwith a 2 508 655 5/1950 silverman 285mg guide channel of the fuelassemblies. A plurality of 3:178356 4/1965 76/50 yoke means, each ofwhlch interconnects at least two 32123979 10/1965 silverblan I 176/36 ofa plurality of control elements situated within some 3,346,459 10 1967Prince et al 176/36 of the hollow tubes, are Provided whereby the inter-3,361,639 1/1968 Ashcroft et a1.. 176/86 connected control elments telescopingly move as a 3,481,832 12/1969 Rickert unit within the guidechannels. 3,595,748 7 1971 Frisch et a1... 3,604,746 9/1971 Notari176/36 13 Claims, 11 Drawing Flgures PATENTEU 1 91974 3.849357 sum 10F 8FIG. I

301191974 PAltNlcu SHE 8 3.849 257 FIG.

FIG.

SHEET 8 OF 8 PATEMLU 1 91974 A M a FIG. 8

PATENTEU 35V 1 91974 SHEET 8 BF 8 FIG.

ll GUIDE STRUCTURE FOR KIONTROIL ELEMENTS BACKGROUND OF THE INVENTIONThis invention relates to nuclear reactors and more particularly to aguide structure for aligning fuel assemblies of a nuclear reactor andfor guiding a plurality of control elements for telescoping movementwithin guide channels of fuel assemblies.

In a nuclear reactor, control elements are provided for insertion intothe core or fuel region of the reactor to control and regulate itsreactivity and power level. These control elements. contain materialsknown as poisons for absorbing neutrons thereby lowering the localneutron flux. Typically, in a pressurized water reactor the controlelements, in being withdrawn and inserted, pass through the outletregion of the reactor in which reactor coolant exiting from the coremust be turned to pass through the outlet nozzles in the side of thereactor vessel. This is necessarily a high cross-flow region due to thehigh flow rates of the reactor coolant passing through the outlet regionand out through the relatively small flow area of the outlet nozzles.Accordingly, if the control elements were not protected in this outletregion, the high flow might cause vibration of the control elementscausing them to wear and possibly fail. In addition, the high lateralloads might cause bowing of the control elements or might result in highfrictional loads of the control elements in their guides, therebypreventing rapid insertion of these control elements in the event of anemergency calling for a reactor shutdown.

In the prior art, the control elements have been grouped or gangedtogether inside the vessel and driven as a unit by a drive mechanismpositioned outside of the vessel. This grouping is necessary due to theconflicting requirements of providing enough control elements in orderto maintain effective control and of allowing a limited number of vesselpenetrations in order to maintain the structural integrity of thevessel. For such systems, protection from the high lateral loads hasnormally been provided by a plurality of shrouds which surround thecontrol elements of a ganged set in the outlet region. I-Ieretofore,such designs have been expensive and have limited the flexibility ofcontrolling the reactor by limiting the geometric arrangement of thecontrol elements. The shrouds necessarily have been massive and of ahigh manufactured quality, and therefore expensive, in order towithstand the high lateral loads. Furthermore, an increase in the numberof control elements, such as grouping more control ele ments togetherwould result in larger shrouds which in turn would increase thehydraulic loads due to a decrease in the flow area in the outlet region.This in turn would result in making the shrouds even more expensive andmassive. Further still, due to the long manufacturing time and the greatnumber of components involved, the specific control pattern for reactormust be chosen at an early date long before actual operation of thereactor. Accordingly, it is difficult to change the specific controlpattern after manufacturing of the reactor has begun.

SUMMARY OF THE INVENTION The present invention overcomes the abovepreviously discussed and other disadvantages of the prior art byproviding a novel guide structure for the control elements. There isprovided a reactor vessel having a nuclear core formed from a pluralityof fuel assemblies, each of which has a plurality of longitudinallyextending fuel elements and at least one guide channel extendinglongitudinally thereof. The guide structure comprises two spaced tubesheets and a plurality of hollo'w tubes extending between and rigidlyconnected thereto. A support means supports the two spaced tube sheetsrelative to the nuclear core with each of the hollow tubes being inalignment with one of the guide channels of the fuel assemblies. Aplurality of control elements are situated in some of the hollow tubesfor telescoping movement within the guide channels in alignmenttherewith. A plurality of yoke means are provided, each of whichinterconnects at least two of the plurality of control elements suchthat the interconnected control elements will telescopingly moveas aunit.

As is apparent from the preceding description, the hollow tubes betweenthe two tube sheets can more easily be designed to withstand thehydraulic loadings thereon since each tube has a smaller area subject tothe loads than the previous massive shrouds. Furthermore, by initiallyproviding extra hollow tubes in alignment with the guide channels of thefuel assemblies the number of control elements providing control of thereactor can be readily increased by simply interconnecting together morecontrol elements which are then driven as a unit by a drive mechanismpositioned outside of the reactor vessel. Further still, bystandardizing the guide structure to provide extra tubes initially, thecontrol pattern of the control elements can be determined after themanufacturing has been completed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is an elevation view partiallyin section illus trating a nuclear reactor embodying the presentinvention;

FIG. 2 is an elevation view partially in section of a portion of thenuclear reactor of FIG. 1 illustrating the guide structure for controlelement assemblies;

FIG. 3 is a vertical cross sectional view illustrating a means ofconnecting the two tube sheets of the guide structure of FIG. 2;

FIG. 4 is a cross sectional view taken along lines 4-4 of FIG. 2illustrating a representative control pattern of a nuclear reactor;

FIG. 5 is a vertical cross sectional view taken along lines 55 of FIG. 4illustrating a four-fingered control element protective shroud employedin the guide structure of FIG. 2;

FIG. 6 is a vertical cross sectional view of a fine grain controlelement shroud employed in the guide structure of FIG. 2;

FIG. 7 is a vertical cross sectional view taken along lines 77 of FIG. 4illustrating an eight-fingered control element protective shroudemployed in the guide structure of FIG. 2;

FIG. 8 is a horizontal cross sectional view of an alternative controlelement assembly and protective guide means;

FIG. 9 is a horizontal cross sectional view of another alternativecontrol element assembly and protective guide means;

FIG. 10 is a vertical cross sectional view of an alternative arrangementof the guide structure employed to distribute emergency core coolant tothe core of the re actor;

FIG. 11 is an elevation view partially in section illustrating anotheralternative arrangement of the guide structure employed to distributeemergency core coolant to the core of the reactor.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, thereis shown a nuclear reactor 20 including a reactor vessel 22 havingtherewithin a core or fuel region 26. The fuel region 26 is comprised ofand defined by a plurality of fuel assemblies 28. These fuel assemblies28 are supported in position by the lower support plate 42 which hasapertures 44 therein for admitting coolant to the reactor Royce J.Rickert. Briefly each fuel assembly 28 includes a plurality oflongitudinally extending fuel ele ments 30 and a plurality of hollowguide tubes 32 interspersed between and within the array of fuelelements 30. In the embodiments shown in the Figures, there are fourguide tubes 32 provided for each fuel assembly 28 although, as it willbe apparent, any number may be utilized. The guide tubes 32 are fixed toupper and lower end plates 34 and 36, respectively, to form the skeletalsupporting structure. I-Iollow alignment-posts 38 are in registry withthe upper ends of the guide tubes 32 and extend upwardly from the fuelassembly end plate 34 and alignment posts 40 extend downwardly from thelower end plate 36 to engage the lower support plate 42.

Control of the neutron flux within the core region 26 is effected in awell-known manner through the use of control elements which contain orare comprised of a neutron poison. Two basic types of control elementassemblies have been depicted in the Figures: primary or shutdowncontrol element assemblies 56, 58 and fine grain or single controlelement assemblies 60. As best seen in FIGS. 1, 2, and 7, the primarycontrol element assemblies 56, 58 comprise a plurality of individual,high worth control elements 102 interconnected or ganged together so asto move as a unit. Two different primary control element assemblies areillustrated: a four-fingered assembly 56 in which the individual controlelements 102 are joined to a four-fingered yoke 96 and an eight-fingeredassembly 58 in which the control elements 102 are joined to aneight-fingered yoke 98. The yokes 96, 98 are each connected to a driverod extension 62 by couplings 100. The drive rod extensions 62 extendthrough nozzles 66 in the pressure vessel head 24 and are connected tocontrol element drive mechanisms 68 mounted outside of the reactorvessel head 24. The control element drive mechanisms 68 are typicallyelectomagnetic linear motion drive devices which move the controlelement 102 in incremental steps into and out of the reactor core 26.

The fine grain control element assemblies 60 can best be seen in FIGS.1, 2, and 6 in which the control elements 104 are of a relatively lowneutron absorption worth and are completely and individually enshroudedabove the core 26 by shrouds or tubing 92. Piping 64 is coupled toshrouds 92 by coupling 63 and passes through a nozzle 66 in the vesselhead 24. Typically, a

plurality of piping 64 passes through one nozzle 66 and connects to ahydraulic actuator assembly 70. The hydraulic actuator assembly 70controls the position of the control elements 104 by exerting a pressuredifferential across the piston head 106 thereof such that the controlelements 104 are either in a fully inserted position or a fullyretracted position. Such a system is described in copending applicationSer. No. 21 1,308 entitled, Top Actuator Reactor Control System by F.Bevilacqua, et. al. filed Dec. 23, 1971 and is advantageous to produceslight variations in the neutron flux of the reactor core region therebyeffecting a finer control of the power distribution.

Located above the core region 26 and spaced slightly therefrom is aguide structure assembly 72 which serves to align the top ends of thefuel assemblies 28 and serves to guide and protect the control elements102 and 104 which enter from above the core region 26. The guidestructure assembly 72 is best seen in FIGS. 1 and 2 and comprises twospaced tube sheets and 82 which are rigidly interconnected to oneanother by means of a plurality of longitudinally extending hollow tubes84. The tubes 84 extend slightly above the upper tube sheet 82 andslightly below the lower tube sheet 80. A support barrel 86 is rigidlyaffixed to the upper tube sheet 82 and supports the two tube sheetswithin the core barrel by means of a flange 87 formed at its upper endwhich rests on the core barrel flange 50.

The guide structure 72 divides the interior of the reactor vessel 22into core region 26, an outlet plenum region 74 and an inactive plenumregion 78 by means of the two tube sheets 80, 82. The significance ofthe outlet and inactive plenum will be discussed hereinbelow. The hollowalignment posts 38 on the upper end plate 34 of the fuel assemblies 28extend into the lower extensions of the tubes 84. The lower tube sheet80 serves as a fuel alignment plate to physically locate the fuelassemblies and as a holddown mechanism to prevent the fuel assemblies 28from being forced upwardly out of position by the reactor coolant. Thisis accomplished by the lower extension of the tubes 84 engagingindividual holddown plates 35 rigidly connected to the alignment posts38 of the fuel assemblies 28. A plurality of control element shrouds 88,90, 92 both for the primary control element assemblies 56, 58 and thefine grain control element assemblies 60 are provided above the uppertube sheet 82 in alignment with the upper extensions of the tubes 84.The control elements 102 and 104 of each of the control elementassemblies extend into the hollow tubes 84 and telescopingly move withinthe control channels formed by the hollow guide tubes 32 and the hollowalignment posts 38 of the fuel assemblies 28. The protective shrouds 88,for the ganged control element assemblies 56, 58 each enshroud thecontrol element assemblies so as to prevent outward bowing of thecontrol elements 102 which might otherwise interfere with effectiveinsertion of adjacent control element assemblies in the event of anemergency. The shrouds 92 for the fme grain control element assemblies60 provide the necessary fluid communication with the hydraulic actuator70 as described above.

In operation, the liquid coolant enters the inlet nozzles 75 and flowsdownwardly around the outside of the core support barrel 48. The coolantthen flows inwardly and up through openings in the lower supportstructure 46 and in the lower support plate 42. As the coolant flowsupwardly through the reactor core 26 it extracts heat generated thereinfrom the nuclear fission in the fuel assemblies 28. The coolant thenflows up through openings (not shown) in the lower tube sheet 88 intothe outlet plenum '74. The coolant next flows outwardly through openings168 in the core support barrel and the outlet nozzles 76 to a heatexchanger (not shown) and, in a conventional manner, passes therethroughand back to the inlet nozzels '75. During normal operation, some of thecoolant flows upwardly through the tubs 84 into the inactive plenum 78and back down to the outlet plenum through openings (not shown) in theupper tube sheet 82. This is necessary to maintain cooling of thecontrol elements and provide proper mixing of the water in the inactiveplenum, and is not considered to be a part of the primary coolant loopas described above. Essentially, the coolant in the inactive plenum 78is stagnant relative to the coolant in the outlet plenum '74.

The outlet plenum 74 is necessarily a high cross flow region since thecoolant must be turned to pass through the outlet nozzle 76. One of theadvantages of the guide structure 72 is that it completely shields thecontrol elements 182, 184 from the adverse effect of this high crossflow in the outlet plenum 74. This is accomplished by the tubes 84 onlyenshrouding one control element each instead of a plurality of controlelements and therefore the tubes 84 may be easily designed to providethe necessary protection. Also, the yoke means 96, 98 which interconnectthe elements 182 of the primary control element assemblies 56, 58 alwaysremains above the upper tube sheet 82 whether the elements 182 are fullyinserted, fully retracted or in some position in between. Thus, the yokemeans 96, 98 need never be subjected to the high cross flow experiencedin the outlet plenum 74.

The detailed construction of the upper guide structure '72 can best beseen in referring to FIGS. 3, 5, 6 and 7. It is apparent that the tubes84 can be rigidly interconnected to the two tube sheets 88, 82 by anywellknown means, such as welding, However, the lower tube sheet 88 isnormally a highly toleranced machined plate and welding of tubes theretomay distort the plate. Thus, in the preferred embodiment a mechanicaljoint formed by two concentric tubes 84, 118 has been utilized. Itshould be understood that only a single tube 84 was illustrated in FIGS.1 and 2 for the sake of clarity and in order to illustrate the broadconcept of the present invention. Referring to FIG. 3, the inner tube isthe control element assembly guiding tube 84 and is provided with aflanged lip H2 at its lower end which engages with the lower surface ofthe lower tube sheet 88. The tube 84 extends upward through the lowertube sheet 88, through the concentric outer tube 118 and through theupper tube sheet 82, terminating a small distance thereabove. Theconcentric outer tube 110 serves as a spacer for the tube sheets 88, 82and engages the lower surface of the upper tube sheet 82 and the uppersurface of the lower tube sheet 88. The decreased diameter 116 and theincreased diameter 117 are provided on the inner surface of the ends ofouter tube 118 to provide a tight fit between the outer tube and theinner tube 84. The ends of outer tube 118 form a quasi seal betweenplates 82 and 88. A plurality of holes 114 are provided in a side wallof the outer concentric tube 118 to prevent air from being trapped inthe annular space between the two concentric tubes 84, 118. A nut 118 isthreaded from above onto the inner guiding tube 8 and screwed downtightly against the upper tube sheet 82, thereby forcing the inner tubes84 to be placed in tension and the outer concentric tube 118 to beplaced in compression. in this way a rigid construction is formedwhereby the lower tube sheet 88 is spaced and supported from the uppertube sheet 82. A lock collar 128 is placed over the nut and crimped intorecesses 124i and T26 on the tube 84 and the nut M8, and then tackwelded to the upper tube sheet 82 such as shown at 122. This rigidlylocks and holds the nut H8 and tubes 84, 118 in place.

Alignment for the alignment posts 38 of all the fuel assemblies 28, eventhose without control elements 102 or 184, is provided by the tubes 84.The number and pattern of tubes 84 is only dependent upon the number ofalignment posts 38 and the pattern of the fuel assemblies 28. In theembodiments shown in the figures, each of the fuel assemblies has fouralignment posts and therefore the tubes 84 of the guide structure 72 arearranged in patterns of four. Thus it is apparent that for anyparticular arrangement of fuel assemblies 28 in the reactor, the guidestructure 72 may be manufactured as a standard independent of thecontrol element pattern to be employed. When a control element patternfor the reactor 28 is finally determined, the guide structure 72 maythen be completed by simply providing an ap propriate protective shroudpattern for enshrouding the control element assemblies.

FIG. 4 shows a representative control pattern for a reactor having 217fuel assemblies 28 in which fourfingered control element assemblies 56,eight-fingered control element assemblies 58, and fine grain controlelement assemblies 68 are utilized. The control element pattern in FIG.4 is only shown for one quadrant of the reactor, the remainder of thecontrol element pattern being symmetric about the two axes 178 andl72.-The eight-fingered control element assemblies 58 are arrangedrelative to the fuel assemblies 28 such that the control elements 182enter three laterally adjacent fuel assemblies 28, two control elements182 each telescoping into the control channels of the outer twoassemblies 28 and four control elements 182 telescoping into the controlchannels of the central fuel assembly 28 positioned between the twoouter fuel assemblies 28. For the four-fingered control elementassemblies 56, each of the control elements 182 telescopes into theguide channels of just one fuel assembly 28. The fine grain controlelement assemblies 68 are interspersed throughout the reactor core witheach control element 184 individually entering into one control channelprovided by a guide tube 32. it should be apparent that with use of theguide structure 72 in a reactor it is possible to arrive at a controlelement pattern wherein all fuel assemblies 28 have control elementstherein. Also, greater flexibility of control can be accomplished with alimited number of nozzle penetrations in the vessel head 2 by utilizinga plurality of eight-fingered control element assemblies 58 wherein thecontrol elements 1182 enter three fuel assemblies 28 rather than justone fuel assembly as in the prior art.

Another advantage of the guide structure 72 is in removing the controlelement assemblies 56, 58 and 60 during refueling of the reactor core26. During refueling, the core 26 is completely flooded with highlyconcentrated borated water which acts as a poison and neutron absorberso that the control elements 102 and 104 may be completely removed fromthe fuel assemblies 28 and the reactor will not become critical. Afterthe head has been removed, the upper guide structure 72 may be removedby a lift rig (not shown) with the control element assemblies 56,58, 60retained therein as a unit. After the fuel assemblies 28 have beenshuffled and/or replaced in the core 26, theguide structure 72 is thenrepositioned in a similar manner with the retained control elementassemblies 56, 58 and 60 providing the same control element pattern asbefore refueling occurred. This is advantageous in that the normalprocedure in refueling is to first remove a fuel assembly 28 with itscontrol element assembly position therein and then switch the controlelement assembly to the new fuel bundle before it is inserted in thereactor core 26.

After a control pattern has been selected for a mac tor, the guidestructure 72 is completed by assembling the control element assemblyprotective shrouds, 88, 90, 92 thereto. Initially, a selected number ofthe shrouds are bolted to the top surface of the upper tube sheet 80 toact as a support for the shroud guide plate 94. In the embodiment shown,these shrouds are the four-fingered control element assembly shrouds 88.As seen in FIG. 5, the protective shrouds 88 are cylindrical in shapeand each have a base plate 138 which has outwardly extending flanges 137which are boltedto the plate 138 has a plurality of openings (not shown)therethrough for receiving the control element guide pin 140 and theupper ends of the tubes 84. Cutouts 139 in the side wall of the shroud88 provide clearance for the tubes 84 within the shroud 88 above thebase plate 138. The guide pin 140 is secured to the top of the uppertube sheet 82 and serves to align the shroud 88 and also the yoke means96 of the control element assembly 56 when the control rods are fullyinserted. The protective shrouds 88 are provided with a plurality offlow openings 148 in the side thereof which allows coolant which comesup through the tubes 84 to exit into the inactive plenum 78. After aplurality of four-fingered control element assembly shrouds 88 aresecured to the top tube sheet 82, the shroud guide plate 94 is loweredover the shrouds 88 and rests on flanges 144 extending outward from theshroud 88. The plate 94 is held in place by bolts 146.

Next, the fine grain cntrol element shrouds 92 (see FIG. 6) are insertedthrough appropriate openings 129 in the shroud guide plate 94 andthreaded onto the upper portion of the tubes 84 extending upward throughthe upper tube sheet 82. An O-ring or equivalent seal 130 is provided atthe coupling joint so as to prevent fluid leakage therethrough. A nut132 is threaded from above onto the upper end of the fine grain controlelement shroud 92 and tightly threaded down until the outwardlyextending flanges 133 on the nut 132 engage the upper surface of theshroud guide plate 94, thereby rigidly supporting the upper end of thefine grain control element shrouds 92. The nut I32 is then tack weldedto the shroud 92 as at 134 to hold it in place.

Finally, the eight-fingered control element protective shrouds (see FIG.7) are lowered through appropriate openings 153 in the shroud guideplate 94. The eight-fingered control element shrouds 90 each have alower base plate 154 integrally attached thereto and a plurality ofopenings (not shown) similar to those for the four-fingered controlelement protective shrouds 88 for receiving the guide pin I40 and theupper ends of the tubes 84. Cutouts 156 .in the side of the shroud 90provide clearance above the base plate 154 for the tubes 84. The shrouds90 rest on the shroud guide plate 94 by means of flanges 158 extendingoutward from the sides of the shrouds 90. The flanges 158 arebolted 0 7the plate 94 by bolts 160. As with the founfingered control elementshrouds 88, a plurality of holes 162 are provided in the side wall ofthe shroud 90 to allow coolant to exit therefrom. The lower end of theeightfingered control element shroud 90 is spaced from the upper tubesheet 82 and held in alignment merely by a tight fit between the upperends of the tubes 84 and the openings in the lower base plate 154.

Each of the nuts and bolts may then be tack welded or otherwise lockedin place so as to prevent loosening thereof during operation of thereactor. The above described procedure of assembly of the shrouds 88,90, 92 to the guide structure 72 is necessitated by the fact that thereis a limited work space in the inactive plenum 78.

Several alternative control schemes are shown in FIGS. 8 and 9 forganged control element assemblies which go to show the great flexibilityof control which is possible with the guide structure 72. FIG. 8 shows al2-fingered control element assembly 174 whose control elements 102 areadapted to enter the guide channels in four fuel assemblies 28. FIG. 8also shows an alternative scheme for guiding and protecting the controlelements 102 above the upper tube sheet 82 from interference of othercontrol element assemblies. Instead of having shrouds, a plurality ofsplit tubes 176 are provided with each having a longitudinal slotextending the length thereof. The split tubes 176 may be affixed to theguide structure in a similar manner as the fine grain control elementshrouds 92. The longitudinal slots are arranged so as to accommodateextensions on the 12- fingered yoke 178 of the control element assembly174 during the vertical travel thereof in the inactive plenum 78. Aplurality of support plates 180 are shown which provide lateral supportfor the split tubes 176.

FIG. 9 shows a six-fingered control element assembly 182 whose yoke 184is shaped in the form of a crows foot. The control elements 102connected to the yoke 184 enter two diagonally spaced fuel assemblies28. A six-fingered control element shroud 186 is affixed to the guidestructure 72 in a similar manner as the shrouds for the preferredembodiment and serves as a guide and protection'means for the controlelements 102 above the upper tube sheet 82.

It should be apparent from the preferred embodiment and the alternativeembodiments that virtually any type of control element pattern in whichsome of the control elements are interconnected to a common yoke can beutilized with this type of upper guide structure. The major feature ofthe guide structure 72 which accommodates this is that the yoke neednever pass into the highcross-flow and turbulent region of the outletplenum 74 in the reactor vessel 22. By substantially isolating theplenum 78 in which the control element yoke travels from the maincoolant path of the reactor it is not necessary to provide massive andheavy shrouding to protect the yoke. Instead, relatively inexpensive orno shrouding may be used to guide the yoke and control elements in theinactive plenum 78 and standardized tubing may be used for each of theindividual control element fingers which pass through the outlet plenum74-. It should however be understood that the protective guidanceshrouding above the upper tube sheet 82 may not be necessary at all. Thefunction the shrouding serves is that of insuring that the controlelement assemblies will not interfere with one another during insertioninto the nuclear core 26. If it can be shown that the elements 102 willnot bow outward if they become stuck in the withdrawn position, then theprotective shrouding is not necessary.

One of the major advantages in using the guide structure '72 deals withcapability of providing a new and novel emergency core cooling system.In the event that the normal cooling is lost or decreased through abreak in the reactor coolant system, it is necessary to provide asufficient coolant flow through the core 26 in order to remove the decayheat which is still being generated in the fuel despite the insertion ofthe control elements 182 and 104 thereinto. Otherwise the fuel mayoverheat and result in fuel cladding failure and release of radioactivecontainments to the atmosphere. Normally, in the case of such anaccident, emergency core coolant water is pumped in through the inletnozzles 75 and allowed to flow down through the annulus between the corebarrel 48 and the vessel 22 as occurs during nor mal operation. As isapparent, there is necessarily a time delay between the startup of theemergency core coolant system and the water reaching the core 26. As thewater begins to fill in the core 26, steam is created which exitsthrough the outlet nozzles 76 as in normal operation of the reactor. Insuch a situation it has been postulated that due to the great amount ofsteam created, steam blockage may occur and thereby reduce theeffectiveness of the emergency core coolant system in removing decayheat.

With the guide structure 72 as previously described, it is possible inthe event of an accident to provide either an alternative emergency corecoolant system or an additional core coolant system which may be used inconjunction wth the prior art systems. FIG. I; shows generally theconcept wherein emergency core coolant water is stored in a tank 188positioned outside of the reactor vessel 22 and which communicates withthe in active plenum region 78 by means of piping 190 pass ing throughthe side wall of the reactor vessel 22 and the side walls of the corebarrel 48 and upper guide structure 72. A valve means 192 is provided inthe piping I98 which is actuated in the event of an accident to allowthe coolant water in the tank I188 to be passed into the inactive plenum78 of the reactor 20. The valve means 192 may be one of any of thewell-known types which are used in actuation of the prior art emergencycore coolant systems. Generally it comprises at least two valves forredundancy in accordance with standard emergency core coolantprocedures. Although only one tank means 188 is shown in FIG. 1, it isapparent that a plurality of tanks 188 and corresponding piping orconduits I90 may be provided for redundancy and additional safety.

It should be noted that a variety of alternatives are available forintroducing the coolant fluid into the inacllll tive plenum 78, all ofwhich may offer additional advantages. For instance; communication tothe inactive plenum 78 could just as easily be provided through thevessel head 24 such as through a nozzle 67 as shown in FIG. lll insteadof through the side wall of the vessel 22. This would be advantageousfor reactors presently in operation. Alternatively, a spray ring headerwith spray nozzles could be utilized, which act as a better condensingmedium for steam in the inactive plenum. If steam can be condensed inthe inactive plenum. the pressure therein will be lowered. This in turnwill act to pull coolant from an inlet nozzle injection system upthrough the core 26. Additionally, the emergency core coolant systemcould be provided with a pumping system for introducing more coolantfluid into the tank means I88 to continue cooling water flow. This isparticularly advantageous in the event there is a break in the-bottom ofthe reactor vessel 22 in which case it would be impossible to fill thecore 26. Instead the additional coolant to the tank I88 would providesufficient cooling of the core 26 till the decay heat has been removed.

As the coolant is introduced in the inactive plenum 78, it flowsdownward by gravity onto the upper tube sheet 82 of the upper guidestructure 72. The guide structure 72 is provided with the capability ofallowing three types of cooling. As shown in FIG. 10, apertures or pipesI96 allow a portion of the emergency core coolant to pass from theinactive plenum 78 into the condensing plenum 74 to outlet in condensingsome of the steam which is passed into the outlet plenum 74. Thecondensed steam will then fall back as water into the core region 26 toaid in cooling the fuel and in refilling the core 28 so that it will becompletely submersed in coolant. Secondly, piping or tubes I94 providedirect communication or flow coupling between the inactive plenum 78 andthe core region 26, thereby bypassing the outlet plenum 74. In FIG. 10,the tubes 194 are similar to the tubes 84 and are positioned cen trallyover a fuel assembly 28 so as not to interfere with i the alignment ofthe fuel assembly alignment posts 38 and tubes 84. The coolant whichflows down these tubes 19 sprays directly onto the fuel assemblies 28which thereby greatly increases the heat removal therefrom. This spraysystem is particularly advantageous in that with normal emergency corecoolant systems, the coolant may not quickly reach the upper fuel regionof the core 26 due to possible steam blockage and the relatively longdelay time after the occurrence of an accident. Finally, cooling ispossible through the tubes 84 which serve to align the alignment posts38 of fuel assemblies 28 and which provide guidance for the controlelement assemblies 56, 58, 60. Since each of the alignment posts 38 andthe guide tubes 32 of the fuel assemblies 28 are hollow to allow forinsertion of control elements I02 and I'M, emergency core coolant may beintroduced into the hollow channels through the tubes 84 and flowdownward to the lower region of the core 26. In this way the emergencycore coolant water bypasses the outlet plenum 7d and a substantialportion of the core region 26. The cooling; of the fuel occursindirectly as the guide tubes are cooled. At least one hole 200 isprovided in each guide tube 32 in the lower re gion of the core 26 toallow coolant to be introduced into the lower portion of the core 26.Additionally. other holes 208 may be introduced along the longitudinallength of the guide tubes 32 to allow for the introduction of morecoolant into the core to directly cool the core 26 and to aid inrefueling. This is particularly advantageous since steam formation inthe core will not adversely affect the flow of the coolant.

Also shown in FIG. 10 is a means for relieving steam in the outletplenum 74. The upper tube sheet is provided with stand pipes 198 whichflow couple the outlet plenum 74 and the inactive plenum 78 to allowsteam in the outlet plenum 74 to be released into the inactive plenum 78where it may be condensed by the emergency coolant water beingintroduced thereinto.

An additional advantage of the emergency core coolant system is that thesequence of cooling operations may be varied simply by varying theelevations of the flow coupling tubing above the upper tube sheet 82. Asthe coolant is introduced into the inactive plenum 74, the elevation ofthe coolant above the tube sheet 82 increases. The sequence of coolingoperation occurs in the order of the elevations of the flow couplingtubing above the tube sheet 82, the operation having its tubingpositioned closest to the tube sheet 82 occurring first. Initially, itis postulated that the preferred sequence of operations would be asshown in FIG. 10. Emergency core coolant is first introduced into theoutlet plenum 74 through tubes 196, then into tubes 194 to spray the topof the core 26 and finally into tubes 84 to indirectly cool the core 26.Thus the elevation of the tubing is such that the top of the tubes 196are closest to the top surface of the upper tube sheet 82, the tubes 194are second closest and the tubes 84 are third closest. The steam reliefstand pipes 198 have their tops located furthest from the top surface ofthe upper tube sheet so as to be able to relieve the most steam from theoutlet plenum 74. As can be seen in FIGS. 6, 7 and 10 each of theshrouds for the ganged control element assemblies (only shroud 90 forthe eight-fingered control element assembly 58 is shown in FIG. 10) areprovided with holes 148, 162 in the side walls thereof to allowemergency core coolant water to enter the shrouds 88, 90 and the tubes84 positioned therein which provide alignment for the alignment posts 38of the fuel assemblies 28 having control elements 102 therein. Obviouslythe elevation of these holes may be varied to facilitate any desiredsequence of cooling operations.

Although only one sequence of cooling operations has been depicted inFIG. 10, it should be evident that the elevations of the tubing mayreadily be changed in order to produce a different sequence of coolingoperations which may prove to be more advantageous in cooling the core26.

FIG. -11 shows an alternative scheme for introducing emergency corecoolant water into the core region 26. This embodiment may be utilizedin conjunction with the prior art systems and/or the system described inFIG. 10. For the alternative embodiment, a plurality of tubes 216 areprovided which pass through the two tube sheets 80, 82 and the shroudguide plate 94 of the guide structure 72. The tubes 216 communicate withthe core region 26 at one end and are coupled to a header 208 at theother end through piping. 212 and couples 214. The header 208 has piping210 which passes through a nozzle 67 in the vessel head 24 and connectsto piping 206 outside of the reactor vessel 22. The piping 206communicates with a tank means 204 having a pressurizing means thereinfor pressurizing emergency core coolant water. A valve means 207,situated in piping 206, is actuatable in the event of an accident tointroduce the pressurized coolant into the header 208. The pressurizedcoolant then flows from the header 208 into the tubes 216 to pressurizespray the core region 26.

The fact that the coolant is pressurized allows the system to overcomeany possible steam blockage problems which might occur in the upperregion of the core 26 and insures that the fuel assemblies 28 will becooled. This is advantageous since otherwise, if steam blockage doesoccur, the gravity head of the emergency core coolant may not besufficient to allow introduction of coolant into the core region 26.Furthermore, with pressurized coolant the problem of steam relief islessened. By introducing coolant fluid into the reactor vessel 22 at apressure higher than the steam, the steam will simply be forced out ofthe reactor through the outlet nozzles 76. As with the embodimentdepicted in FIG. 10, redundancy and additional safety may be insured byproviding additional tanks and pressurizers 204 and piping 206 whichcommunicate with the header 208. Also a plurality of headers 208 couldbe provided. Further still, additional or alternative piping from theheader 208 could be utilized to provide the cooling operations depictedin FIG. 10.

While preferred embodiments of the invention have been shown anddescribed, it will be understood that these are merely illustrativerather than restrictive and that changes may be made without departingfrom the invention as claimed.

What is claimed is:

1. A nuclear reactor comprising, in combination:

a vessel having a nuclear core fixedly positioned therein;

a plurality of fuel assemblies comprising said nuclear core, each ofsaid fuel assemblies having a plurality of longitudinally extending fuelelements and a plurality of longitudinally extending guide channels;

an upper plate and lower plate in said vessel, said lower plateoverlying all of said plurality of fuel assemblies and said upper platebeing spaced above said lower plate;

plurality of hollow tubes extending between and rigidly connected tosaid upper plate and said lower plate, the wall of each of said hollowtubes being circumferentially continuous substantially over the lengththereof, and each of said hollow tubes being in alignment with only oneof said guide channels;

a support means for supporting said two spaced plates in said vessel; aplurality of control elements each of which is situated for telescopingmovement within only one of said hollow tubes and said channel inalignment therewith; plurality of yoke means each of which is whollywithin said vessel and which interconnects at least two of saidplurality of control elements, each of said yoke means having aplurality of laterally extending fingers each of which overlies and isin alignment with one of said hollow tubes and which is connected tosaid control element situated therein; connection means for each of saidyoke means connected to said yoke means inside said vessel and passingoutside of said vessel, each of said interconnected connection means,yoke means and control elements forming a rigid and essentiallynonflexible unitary structure; and drive means positioned outside ofsaid vessel for each of said connection means, each of said drive meansbeing connected to one of said connection means for driving said controlelements connected thereto between a fully inserted position and a fullyretracted position to control the reactivity of said nuclear core duringnormal operation, the position of each of said yoke means for both ofsaid control element positions being vertically above both of said upperand lowerplates.

2. The combination of claim 1 further including a plurality ofprotective guidance means above said upper plate, each of saidprotective guidance means guiding said control elements interconnectedto one of said yoke means so as to prevent interference between saidcontrol elements interconnected to said one of said yoke means and theother of said control elements and said yoke means.

3. The combination of claim 2 wherein said protective guidance means areshrouds each of which enshrouds one of said yoke means and said controlelements interconnected thereto.

4. The combination of claim 3 wherein said shrouds each have flowopenings on the side walls thereof.

5. The combination of claim 2 wherein said protective guidance means area plurality of longitudinally extending split tubes, each of said splittubes having a longitudinal slot along the length thereof to allowpassage of said laterally extending finger connected to said con trolelement for which said tube is providing guidance during vertical travelof said finger.

6. The combination of claim 1 wherein said fuel as semblies each havealignment means, and wherein each of said alignment means has one end inregistry with one of said channels of said fuel assemblies and has itsother end received within and positioned by one of said hollow tubes.

7. The combination of claim 6 wherein:

said reactor vessel includes a lip;

said support means includes a cylindrical barrel integral with one ofsaid upper and lower plates; and

said barrel includes outwardly extending flanges which rest on said lipof said reactor vessel.

8. The combination of claim 1 wherein at least one of said yoke meanshas laterally extending fingers which are connected to a plurality ofsaid control elements which telescopingly move into said channels of aplurality of said fuel assemblies.

9. The combination of claim 7 wherein each of said hollow tubes extendsbetween and through each of said upper and lower plates one end of eachof said hollow tubes having flanges engaging one of said upper and lowerplates and the other end being threaded; and

wherein there is provided a nut and an outer concentric tube for each ofsaid hollow tubes, each of said outer concentric tubes being positionedaround one of said hollow tubes and between said upper and lower platesand each of said nuts being threaded onto said threaded end of saidhollow tubes whereby each of said hollow tubes is placed in tension andeach of said outer concentric tubes is placed in compression.

10. The combination of claim 8 wherein at least one of said controlelements of said plurality is free for individual telescoping movementwithin one of said hollow tubes and said channel in alignment therewithindepen dent of the remainder of said control elements.

11. The combination of claim 10 wherein said one yoke meansinterconnects a plurality of said control elements which telescopinglymove into said channels of two of said fuel assemblies.

12. The combination of claim 10 wherein said one yoke meansinterconnects a plurality of said control elements which telescopinglymove into said channels of three of said fuel assemblies.

13. The combination of claim 10 wherein said one yoke meansinterconnects a plurality of said control elements which telescopinglymove into said channels of four of said fuel assemblies.

1. A nuclear reactor comprising, in combination: a vessel having anuclear core fixedly positioned therein; a plurality of fuel assembliescomprising said nuclear core, each of said fuel assemblies having aplurality of longitudinally extending fuel elements and a plurality oflongitudinally extending guide channels; an upper plate and lower platein said vessel, said lower plate overlying all of said plurality of fuelassemblies and said upper plate being spaced above said lower plate; aplurality of hollow tubes extending between and rigidly connected tosaid upper plate and said lower plate, the wall of each of said hollowtubes being circumferentially continuous substantially over the lengththereof, and each of said hollow tubes being in alignment with only oneof said guide channels; a support means for supporting said two spacedplates in said vessel; a plurality of control elements each of which issituated for telescoping movement within only one of said hollow tubesand said channel in alignment therewith; a plurality of yoke means eachof which is wholly within said vessel and which interconnects at leasttwo of said plurality of control elements, each of said yoke meanshaving a plurality of laterally extending fingers each of which overliesand is in alignment with one of said hollow tubes and which is connectedto said control element situated therein; connection means for each ofsaid yoke means connected to said yoke means inside said vessel andpassing outside of said vessel, each of said interconnected connectionmeans, yoke means and control elements forming a rigid and essentiallynonflexible unitary structure; and drive means positioned outside ofsaid vessel for each of said connection means, each of said drive meansbeing connected to one of said connection means for driving said controlelements connected thereto between a fully inserted position and a fullyretracted position to control the reactivity of said nuclear core duringnormal operation, the position of each of said yoke means for both ofsaid control element positions being vertically above both of said upperand lower plates.
 2. The combination of claim 1 further including aplurality of protective guidance means above said upper plate, each ofsaid protective guidance means guiding said control elementsinterconnected to one of said yoke means so as to prevent interferencebetween said control elements interconnected to said one of said yokemeans and the other of said control elements and said yoke means.
 3. Thecombination of claim 2 wherein said protective guidance means areshrouds each of which enshrouds one of said yoke means and saId controlelements interconnected thereto.
 4. The combination of claim 3 whereinsaid shrouds each have flow openings on the side walls thereof.
 5. Thecombination of claim 2 wherein said protective guidance means are aplurality of longitudinally extending split tubes, each of said splittubes having a longitudinal slot along the length thereof to allowpassage of said laterally extending finger connected to said controlelement for which said tube is providing guidance during vertical travelof said finger.
 6. The combination of claim 1 wherein said fuelassemblies each have alignment means, and wherein each of said alignmentmeans has one end in registry with one of said channels of said fuelassemblies and has its other end received within and positioned by oneof said hollow tubes.
 7. The combination of claim 6 wherein: saidreactor vessel includes a lip; said support means includes a cylindricalbarrel integral with one of said upper and lower plates; and said barrelincludes outwardly extending flanges which rest on said lip of saidreactor vessel.
 8. The combination of claim 1 wherein at least one ofsaid yoke means has laterally extending fingers which are connected to aplurality of said control elements which telescopingly move into saidchannels of a plurality of said fuel assemblies.
 9. The combination ofclaim 7 wherein each of said hollow tubes extends between and througheach of said upper and lower plates one end of each of said hollow tubeshaving flanges engaging one of said upper and lower plates and the otherend being threaded; and wherein there is provided a nut and an outerconcentric tube for each of said hollow tubes, each of said outerconcentric tubes being positioned around one of said hollow tubes andbetween said upper and lower plates and each of said nuts being threadedonto said threaded end of said hollow tubes whereby each of said hollowtubes is placed in tension and each of said outer concentric tubes isplaced in compression.
 10. The combination of claim 8 wherein at leastone of said control elements of said plurality is free for individualtelescoping movement within one of said hollow tubes and said channel inalignment therewith independent of the remainder of said controlelements.
 11. The combination of claim 10 wherein said one yoke meansinterconnects a plurality of said control elements which telescopinglymove into said channels of two of said fuel assemblies.
 12. Thecombination of claim 10 wherein said one yoke means interconnects aplurality of said control elements which telescopingly move into saidchannels of three of said fuel assemblies.
 13. The combination of claim10 wherein said one yoke means interconnects a plurality of said controlelements which telescopingly move into said channels of four of saidfuel assemblies.